The present invention relates to paperboard packaging material and packages constructed therefrom, and more particularly to packaging useful for heating and browning foods in a microwave oven, commonly known in the trade as susceptor packaging.
One of the problems associated with the use of microwave energy for cooking is that is fails to brown and crisp those foods which are normally expected to have such a quality. Many attempts have been made to correct this deficiency including modifications to the microwave oven, the development of new cooking utensils, and more recently, the development of susceptor packaging to solve the browning problem. The most commonly available susceptor packaging on the market today involves the use of metallized films as the susceptor material which are incorporated into the food package, or added as inserts into the food package. The metallized films are generally laminated to the packaging material used to make the food packages, or to the substrates used as inserts. Unfortunately, the use of packages or inserts containing metallized film have only met with limited success. U.S. Pat. Nos. 4,230,924; 4,267,420; and 4,641,005 are typical of those which disclose the use of metallized films in the packaging material.
Another method for adding the susceptor material to the packaging material involves a hot stamping transfer process as taught by U.S. Pat. No. 4,676,857. In this patent, aluminum roll leaf is hot stamped in a variety of patterns onto trays or the like in which the food is cooked. However, this process is tedious and requires specialized equipment.
In contrast to the aforementioned methods of manufacturing susceptor packaging, the present invention comprises a printing process using conventional printing techniques and a solvent based ink vehicle for applying a susceptor material such as conductive carbon, in the form of carbon black or graphite, in selected locations on the packaging material. The use of particulate conductive carbon as a microwave absorber is taught by U.S. Pat. No. 4,518,651, but in that patent, the carbon is dispersed generally in a laminated composite material which makes it no more useful than a metallized film laminate. In addition, U.S. Pat. No. 2,014,760 teaches a printing ink containing graphite, but there is no suggestion that the ink is conductive. Meanwhile, in European Patent Application EP 0 276 654, a susceptor film is disclosed comprising a cross linking and heat resistant synthetic binders which contains evenly distributed particles such as natural and synthetic graphite particles and carbon black particles. An aqueous system is disclosed which may be applied in a printing step as a continuous layer or only in discrete areas. However, according to the present invention, the conductive carbon susceptor material is dispersed in a solvent susceptor-ink composition for application to the packaging material using conventional ink technology.